1. Field of the Invention
This invention relates to a novel single-crystal nickel-based superalloy with high temperature strength, hot corrosion resistance and oxidation resistance.
2. Description of the Prior Art
Recently, power engines such as jet engines and gas turbines have necessarily been used at high turbine inlet temperatures to increase their performance and efficiency. Therefore, we inventors have thought it is very important to develop turbine blade materials that are resistant to such high temperatures.
Major properties required for the turbine blade materials are high creep rupture strength and ductility that can withstand centrifugal forces at high temperatures and high oxidation resistance and hot corrosion resistance to hot combustion gas atmosphere. To satisfy these requirements, nickel-based single-crystal superalloys have been expected hopefully and have stepped toward the practical use.
Unlike conventional casting alloys (ex-axis grain) and unidirectional solidification alloys, the single-crystal articles of nickel-based superalloy have no grain boundaries and can be solution-treated just under the solidus temperature. Therefore, we can get a uniform macrostructure completely free from the segregation. The nickel-based single crystal superalloys have much higher in the creep rupture strength and ductility than the conventional cast nickel based superalloys. The solution heat-treatment just under the solidus temperature allows the alloys to contain more solution hardening elements than the conventional cast alloys. Namely, single crystal superalloys have a feature of containing a lot of W and Ta which have high solution hardening abilities and increasing the creep rupture strength.
Japanese Application Laid-open Publication No. 07-138683 discloses a single-crystal nickel-based superalloy comprising by weight, 1.8 to 4.0% Cr, 1.5 to 9.0% Co, 3.5 to 7.5% W, 5.0 to 7.0% Re, 7.0 to 10.0% Ta, 0.1 to 1.2% Ti, 5.0 to 7.0% Al, 0.25 to 2.0% Mo, 0 to 0.5% Nb and 0 to 0.15% Hf.
In recent years, single-crystal nickel-based superalloys as disclosed by Japanese Application Laid-open Publication No. 07-138683 has been developed actively and a lot of alloys have been invented. However, these alloys have been developed focusing mainly on improving their creep rupture strength at high temperatures, whereas their hot corrosion resistance and oxidation resistance have been considered little.
Increasing the content of Cr or Re is most effective in improving the hot corrosion resistance of the alloy. However, a large amount of chromium (Cr) in the alloy reduces the solubility limit of elements such as W and Ta having high solution hardening abilities and cannot increase the creep rupture strength of the alloy. In contrast to Cr, rhenium (Re) improves the hot corrosion resistance but drastically reduces the oxidation resistance of the alloy at high temperatures. Therefore, all of these properties (high temperature strength, hot corrosion resistance and oxidation resistance) are not satisfied in nickel-based superalloys developed so far.